One day, Spenser was reading Chaucer, and found a passage in which Troilus is described as being second to no one in "derring do that longeth to a knight." In other words "in daring to do that which belongs to a knight." Spencer thought he had discovered a new figure of speech, "derring-do." He assumed it meant something along the lines of "desperate courage," I don't know why, and, without having done any research on the validity of the term, started using it in his books.